Some Escherichia coli carry plasmids which confer on the host bacterium the ability: (1) to produce an antibiotic-like protein, called colicin, whose adsorption to specific bacterial receptors leads to cell death, and (2) to survive treatment with homologous colicin, called immunity. To further our understanding of colicin action, we are undertaking a program (1) to study the events associated with mitomycin C induced synthesis of colicin I, (2) to isolate and characterize the E. coli colicin I specific receptor as well as to study its interaction with colicins, and (3) to correlate the structure of colicins with their biological specificities. BIBLIOGRAPHIC REFERENCES: Gilchrist and Konisky. J. Biol. Chem. 250:2457 (1975). Konisky. J. Bacteriol. (1975, in press).